Cadmium and, in particular, cadmium telluride (CdTe) are gaining a substantially increased role in global energy production and are a component of many solar panels. The environmental safety concern of cadmium leaching out of CdTe solar panels has become a critical issue. Recent studies show that naturally occurring groundwater and rainwater dissolve CdTe at a very fast rate, allowing cadmium to enter the groundwater at concentrations at least four orders of magnitude greater than the EPA allowed limits for drinking water.
Recent studies have also demonstrated that water travels by capillary action between the various layers in a typical photovoltaic (PV) module structure, thus leading to delamination and contamination by cadmium at an even faster rate than has otherwise been anticipated. The behavior of the cadmium leaching out of CdTe PV solar panels makes it imperative not to discard rejected CdTe solar modules/panels in dumps or landfills.
CdTe-based photovoltaic modules have gained a market penetration of approximately ten percent of the existing PV market. In 2010, this proliferation represented an annual manufacturing capacity of approximately two gigawatts and an installed capacity by the end of 2010 of approximately five gigawatts. Unless further manufacture of CdTe is curtailed, a cumulative installed capacity of approximately 50 gigawatts is possible by 2020. This increase means that 100,000 hectares or 1,000 square kilometers would be covered with this cadmium containing photovoltaic material. Approximately 5,000 tons of water-soluble carcinogenic CdTe would thereby be spread over the 1,000 square kilometers of land area.
Cadmium has been established by several international agencies as one of the most carcinogenic materials. The “Restrictions of Hazardous Substances” Directive (RoHS) was signed by many nations including the European Community, China, Japan, and India. In the European Community, however, the promoters of CdTe have managed to get an exemption from the Directive for CdTe photovoltaics. Some other countries, such as Japan, China, and India, continue to ban CdTe. In Europe and the United States, CdTe-based PV modules are heavily marketed. In 2010, for example, the industry sold approximately 2 gigawatts of CdTe-based modules. The initial exemption in Europe depended heavily on white papers published in the United States by NREL personnel and some other related institutions, such as Brookhaven National Laboratory. Those papers claimed that CdTe is not water-soluble. Also, the manufacturers of the modules declared that the installed modules would be dismantled at the end of their useful live (20-30 years) and that all cadmium would be subsequently reclaimed. To pay for this cost, the manufacturer sets aside approximately $0.015 per watt. Keeping in mind the magnitude of the problem and the danger to humans and other living things, these two fundamental assumptions have been examined: the water solubility of cadmium from modules, and the cost and responsibility for recycling.
The makers of CdTe photovoltaic modules do not acknowledge the toxic and carcinogenic properties of cadmium and have therefore promised that when the modules reach the end of their useful life they will not de-mount them, pack them, ship them “home” and salvage the cadmium. Some companies have set up funds which are supposed to cover the cost of doing so even though that cost would nearly equal that of delivering and installing the modules in the first place. History has shown us, though, that such promises, however vociferously advertised, are not always fulfilled, especially when they involve projections of 25 to 30 years into the future.
A method of efficiently removing the cadmium from the PV modules is thus needed to aid in the prevention of cadmium-based pollution.
U.S. Pat. No. 5,405,588 generally describes a process for the recovery of a metal, in particular, cadmium contained in scrap, in a stable form. The process comprises the steps of mixing the cadmium-containing scrap with an ammonium carbonate solution, preferably at least a stoichiometric amount of ammonium carbonate, and/or free ammonia, and an oxidizing agent to form a first mixture so that the cadmium will react with the ammonium carbonate to form a water-soluble ammine complex; evaporating the first mixture so that ammine complex dissociates from the first mixture leaving carbonate ions to react with the cadmium and form a second mixture that includes cadmium carbonate; optionally adding water to the second mixture to form a third mixture; adjusting the pH of the third mixture to the acid range whereby the cadmium carbonate will dissolve; and adding at least a stoichiometric amount of sulfide, preferably in the form of hydrogen sulfide or an aqueous ammonium sulfide solution, to the third mixture to precipitate cadmium sulfide. This mixture of sulfide is then preferably digested by heating to facilitate precipitation of large particles of cadmium sulfide. The scrap may be divided by shredding or breaking up to expose additional surface area. Finally, the precipitated cadmium sulfide can be mixed with glass formers and vitrified for permanent disposal.
Examples of related art are described below:
U.S. Pat. No. 4,370,233 generally describes a method for the chemical detoxification of anaerobically digested organic sludge containing toxic heavy metals in insoluble form. A quantity of the sludge is transferred from a conventional anaerobic digester to an insulated reactor vessel where the sludge is mixed and aerated at a rate sufficient to raise the oxidation reduction potential of the sludge to above +300 mv. and to maintain this condition for a period of 6-12 hours during which the heavy metals are converted to their desired oxidation state. The sludge is then acidified under controlled conditions to pH 1.0-3.0 for a period of 6-12 hours to solubilize the heavy metals. Conventional dewatering techniques are used to separate the detoxified, acidic sludge and the acidic, heavy-metal-containing water. The sludge may be neutralized for safe land application, and the metals can be recovered from the water using existing conventional techniques.
U.S. Pat. No. 5,997,718 generally describes a method for extracting and reclaiming metals from scrap CdTe photovoltaic cells and manufacturing waste by leaching the waste with a leaching solution comprising nitric acid and water, skimming any plastic material from the top of the leaching solution, separating the glass substrate from the liquid leachate and electrolyzing the leachate to separate Cd from Te, wherein the Te is deposits onto a cathode while the Cd remains in solution.
U.S. Patent Publication No. 2009/0095127 generally describes a system and process for reclaiming nickel and cadmium from a feed source such as Ni—Cd batteries. The feed source is shredded to produce feed particles, screened to size the particles, magnetically separated to remove non-metallic materials, and induction heated to generate nickel and cadmium products.
None of the art described above addresses all of the issues that the present invention does.